1. Field of the Invention
The present invention relates to a transmitter and receiver apparatus (transceiver) and in particular relates to an initial training apparatus for use in an echo canceler transmission apparatus for correcting an echo signal in the best way by using an adaptive gain of an automatic equalizer.
2. Description of the Related Art
Equipment for transmitting and receiving signals is used to take advantage of the recently introduced bidirectional digital transmission method.
FIG. 1 shows a system structure based on digital subscriber line transmission. In FIG. 1, 1 designates a terminal apparatus provided within a subscriber's premises and 2 a transmission apparatus (transmitter). The subscriber's premises are connected to other transmission apparatuses in a station by a subscriber line 3, which is a two-wire metallic cable. This cable is used for bidirectional digital transmission, and provides a data capacity of 160 kb/s. A structural view of a transmission apparatus for a subscriber's premises is shown in FIGS. 2A and 2B. Generally, the transmission apparatus needs an automatic equalizer EQL to compensate for a waveform distortion on a transmission line.
Further, some transmission apparatuses include an echo canceler for suppressing echo leaking from a transmitted signal, and some do not. FIG. 2A shows a structural view of a transmitting apparatus based on a ping-pong transmission method, which does not use an echo canceler, and FIG. 2B shows a structural view of a transmission apparatus based on an echo cancelling transmission method, which does use an echo canceler. At present, a ping-pong transmission system is used in Japan and an echo cancelling transmission system is used in Europe and North America.
In the ping-pong transmission system, as illustrated in FIG. 2A, the data capacity for transmission is 160 kb/s, but as bidirectional transmission is used in a time divisional manner, the data capacity on subscriber line 3 is doubled, to 320 kb/s. Transmitting or receiving is switched by a transmitting and receiving switch SW. Transmission data is coded by a coding circuit, (coder) COD, and are driven by a line driver DRV. At this time a transmission switch SW selects a transmission site to transmit a signal on a subscriber line 3. On the other hand, a received signal transmitted from subscriber line 3 is selected by a transmitting and receiving switch SW, and then enters line equalizer EQL. Line equalizer EQL includes .sqroot.F AGC (automatic gain control) and automatically controls gain lost in a transmission line of a subscriber to automatically correct a gain matched with a frequency. The output of line equalizer EQL is applied to decoder DEC, thereby providing received data. In this case, timing reproducing circuit TIM reproduces a clock signal, namely, a synchronizing signal from a received signal and decodes the received data by using the clock. This ping-pong transmission method uses bidirectional signals on a transmission line in a time divisional manner. Thus, when transmission data are transmitted outwardly, they do not leak into the receiving side. Therefore, this system does not need an echo canceler. On the other hand, the data capacity of the transmission line becomes double the demanded data capacity of 160 kb/s, namely 320 kb/s. Therefore, a smaller hardware is required, but the transmission rate is lowered, so that there is a trade-off relationship.
FIG. 2B shows the structure of an echo canceler transmission system. Transmission data are coded by a coding circuit COD and are transmitted on subscriber line 3 through line driver DRV. In an echo canceler transmission system, data capacity of a transmission on subscriber line 3 is 160 kb/s and it does not perform bidirectional time divisional multiplexing. Then, transmitted and received signals are multiplexed in an analog manner. A received signal is inputted to hybrid circuit HYB i.e. a 2-wire/4-wire conversion circuit, from subscriber line 3. Where a transmission signal is transmitted on subscriber line 3 through hybrid circuit HYB, a part of it leaks as an echo, and is turned into its own apparatus on the receiving side. Therefore, in order to cancel an echo signal, an echo canceler EC is added. Echo canceler EC has, for example, a transversal filter structure. A pseudo echo (echo replica) is artificially formed as an impulse response to a transmitted signal. Coefficients of the transversal filter are determined by the echo signal, thereby producing the echo replica.
The echo replica is subtracted from the echo signal which leaks into the receiving side, thereby cancelling it. The received signal is thereby input into line equalizer EQL in which a gain for compensating a loss of a transmission line is automatically formed. The received signal is automatically multiplied by the gain, and the timing reproduction circuit extracts a synchronizing clock signal from it. A decoding circuit DEC decodes the received signal based on the synchronizing clock signal. This echo canceler transmission method needs an echo canceler and thus increases the volume of hardware. However, the transmission capacity of a subscriber line remains the same as that of the data capacitor, i.e. 160 kb/s. Thus, a time division multiplexing is not necessary for transmission. The transmission apparatus of an echo canceler transmission method includes a line equalizer and an AGC circuit to compensate for a waveform distortion on the transmission line and an echo canceler for suppressing leaking echoes of the transmitted signal. Before starting a normal data transmission, and upon an initial training in which a synchronization is established between transmitting and receiving sides, the gain and the coefficient of an echo canceler apparatus are determined. In other words, the initial training of the transmission apparatus comprises a training by an echo canceler and a training by an AGC. The performance of the transmission apparatus depends on how the initial training is conducted.
FIG. 3A shows the transmission apparatus which uses an echo canceler method. The apparatus includes the above automatic equalizer and echo canceler. The transmission apparatus comprises an echo canceler EC, automatic equalizer EQL, hybrid circuit HYB, subscriber line 3, a coding circuit COD and A/D converter A/D.
The apparatus shown in FIG. 3A performs an echo cancellation before automatic equalizer EQL. The apparatus shown in FIG. 3B relates to a method of performing an echo cancellation after the automatic equalizer EQL. The apparatus shown in FIG. 3C relates to a method of performing an echo cancellation both before and after the automatic equalizer EQL.
As shown in FIG. 3A, when an initial training is conducted, a pulse signal is transmitted from the apparatus to a transmission line through hybrid circuit HYB. This pulse signal trains an echo canceler EC, and an echo formed by a transmission pulse through a hybrid circuit is input to a subtraction circuit after it is subjected to A/D conversion. The coefficients of the EC is changed such that the difference between the output from the EC and the echo signal included in it decreases, thereby forming an echo replica. After the echo is canceled in this manner, equalizer EQL amplifies the received signal by the gain lost through the transmission line. If echo canceler EC and automatic equalizer EQL are all formed by an analog circuit, their control becomes difficult. Thus, in this structure, an A/D converter is provided at the output of hybrid circuit HYB. Therefore, various digital processes are conducted in a digital manner, and thus, the A/D converter must have about 13-bit accuracy. Therefore, the load on the digital circuit increases. After training of the echo canceler is completed, signals transmitted from another apparatus via the same circuit through a transmission line is received by hybrid circuit HYB, thereby enabling automatic equalizer EQL to be trained for a line equalization. After a predetermined time period, training of the automatic equalizer is completed. This method follows a rule that a training of echo canceler EC is conducted prior to that of automatic equalizer EQL.
On the other hand, when the apparatus shown in FIG. 3B is trained, an echo canceler is positioned after the automatic equalizer. This apparatus is suitable for a rule in which a gain training is first conducted by an automatic equalizer in a way similar to that described above and thereafter, an echo training is conducted by using the echo canceler. If this rule is followed, the amplitude of the echo signal is determined by the gain of the automatic equalizer, and this kind of echo canceler performs a better cancellation than that of the apparatus shown in FIG. 3A. However, this is not applied to a rule in which an echo canceler training is conducted prior to an automatic equalizer training.
The apparatus shown in FIG. 3C performs echo cancellation both before and after the automatic equalizer. In this case, the A/D converter is positioned after the automatic equalizer. Thus, conversion accuracy of the A/D converter can be decreased. Echo cancelling is conducted before the equalizer EQL through analog processing and an A/D converter of about 10 bit accuracy is sufficient, thereby decreasing the load on the digital circuit and facilitating manufacture of LSIs.
The transmission apparatus shown in FIG. 4 comprises a master-side transmission unit 10 and an opposite slave-side transmission unit 11, and respective units comprise source oscillators 12 and 13, whose oscillation frequencies are divided to determine the desired transmission speed.
An automatic equalizer provided within a transmission apparatus, as shown in FIG. 5, is known. The analog output of analog line equalizer 15, which produces a gain characteristic of a sufficient value upon receiving an input signal, is sampled and converted to a digital signal by A/D converter 16, thereby providing a sampled value X.sub.n of the received signal. Power arithmetic operation circuit 17 performs an arithmetic operation to produce an average value E(Xn.sup.2) of the power of a digitally sampled value Xn output from A/D converter 16. AGC circuit 18 performs an arithmetic operation to determine the most appropriate gain. The sampling frequency applied to A/D converter 16 can be obtained by the dividing clock frequency f.sub.m of source oscillators 12 and 13 (as shown in FIG. 4).
Therefore, AGC circuit 18 can determine the most appropriate adaptive gain after training.
The method shown in FIG. 3A is disadvantageous in that a residual echo, which cannot be canceled by echo canceler EC, is amplified by a filtering characteristic of the automatic equalizer EQL, so that it appears as the received signal, if the accuracy of the A/D converter is decreased. Therefore, in order to avoid the above defect, it is necessary to increase the accuracy of the A/D converter.
The methods disclosed in FIGS. 3B and 3C can achieve an echo cancellation of the output from the automatic equalizer EQL as the training of echoes is conducted after the training of equalizer EQL. However, in this method, the order of training is predetermined and if it is not satisfied, the most appropriate training cannot be conducted, under a rule that the training of equalizer EQL is conducted after the training of echoes.
In FIG. 3C, the echo cancelers are provided before and after the equalizer EQL. However, the echo canceler provided after the equalizer EQL cannot conduct the training of the echo canceler before that of the equalizer EQL. In other words, the echo canceler is used under a rule that the training of the equalizer is conducted prior to that of the echo canceler.
Such initial training of an automatic equalizer is conducted at the beginning of the initial training of a transmission apparatus, and in most cases when timing (phase) data of a received signal has not yet been obtained.
In order to perform AGC training during a period when timing data does not exist, it is necessary to perform an A/D conversion of a received signal at a sampling frequency Nxf.sub.s which is more than twice the received signal frequency f.sub.s (transmission rate).
The great barrier is that the automatic equalizer installed with the A/D converter has a high sampling frequency and is made of LSIs. This inhibits the development of an inexpensive, compact and highly reliable transmission apparatus. The transmission speed increases with the expansion in transmission capacity, as has been observed recently. This is a further difficulty.